Thousands of art and antiques dealers, fair organisers and auction houses have been targeted in a global extortion scam that involves victims being subjected to months – and even years – of threatening demands for money.

Unsuspecting British and foreign traders have been conned by what appear to be bona fide forms – either sent by email or posted – merely purporting to check whether their business details in a "free" international trade listing guide are correct. Only after sending off the form do victims discover that the small print commits them to paying thousands of pounds for placing an "advertising order" in the guide.

Those who are caught out are pursued relentlessly, enduring intimidating threats by letters or phone calls to pay up or face prosecution and debt collectors.

The art trade is not alone. Great numbers of people from other business sectors have been conned. The European Commission is now so alarmed that it has launched an investigation, calling for evidence to be submitted by 16 December.

Ivan Macquisten, editor of industry newspaper the Antiques Trade Gazette, which was itself targeted by the scam last week – told the Observer about the fear experienced by victims: "It has affected, personally, thousands of lives. It is a huge problem, but one that lots of people have never heard of."

The fraudsters trade under different company names in different countries and, if the authorities catch up with them, they pop up again somewhere else. Many victims do not realise there is no legal basis for paying or are reluctant to incur legal advice, and are embarrassed into silence. Most recently, the art trade has been targeted by companies apparently based in Spain and Mexico.

The victims include Ian Butchoff, a dealer in antique furniture in London. "I wouldn't think there's anybody in the trade who hasn't been targeted at one time or another. How many people have paid is another matter," he said.

The form that he received looked "very official", he said. It was followed by a letter "confirming" his annual €700 (£600) payment, which he was advised to ignore by a solicitor friend because the company, based in Austria, had no jurisdiction under English law. "But they kept harassing me … with official-looking solicitors' letters from Switzerland, which were very menacing. They eventually went away."

His wife, who has a framing business, was also targeted and paid up "because they were threatening her", Butchoff said. When the conmen came back for more money, she refused, and eventually they went away.

Butchoff said: "I still get letters … from different companies. Now they're asking for €12-13,000 a year."

Macquisten has put together a huge dossier of evidence. "Thousands of people have been targeted … and that's just in our industry," he said.

"It is a global problem involving huge sums of money. They are banking on enough people being scared into paying."